1. Field of the Invention
The present invention relates to a method for producing of aromatic compounds having an alkyl group with at least 3 carbon atoms, which are useful as starting materials for production of various pharmaceuticals and agricultural chemicals, through conversion, isomerization and/or adsorptive separation of aromatic compounds, and also to catalysts and adsorbents for the method.
2. Description of the Related Art
In aromatic compounds having a branched alkyl group with at least 3 carbon atoms, in general, it is difficult to change the positions of the carbon atoms of the alkyl group that is bonded to the aromatic ring. Especially, in those, compounds the steps of (A) reducing the number of the branches of the alkyl group, (B) shortening the branched side chains of the alkyl group and (C) changing the alkyl group into a different one that is bonded to the aromatic ring via a secondary carbon are difficult.
One example of changing the position of the carbon atoms of an alkyl group bonding to an aromatic ring, thereby reducing the number of the branches of the alkyl group is represented by the following chemical reaction formula: 
wherein
R1 to R5 each represents a methyl, an ethyl group, or a linear alkyl group with at least 3 carbon atoms;
X1 and X2 each represent a methyl group, an ethyl group, a halogen atom, a formyl group, a carboxyl group, an alkoxy group, a nitro group, an amino group, or a cyano group;
n is from 0 to 5.
Another example of changing the position of the carbon atoms of an alkyl group bonding to an aromatic ring, thereby shortening the branched side chains of the alkyl group is represented by the following chemical reaction formula: 
wherein
R6 represents an ethyl group, or a linear alkyl group with at least 3 carbon atoms;
R8 represents a hydrogen atom, or an alkyl group of which the carbon chain is shorter than that of R6;
R7 and R9 each represents an alkyl group;
X3 and X4 each represent a methyl group, an ethyl group, a halogen atom, a formyl group, a carboxyl group, an alkoxy group, a nitro group, an amino group, or a cyano group;
n is from 0 to 5.
Still another example of changing the position of the carbon atoms of an alkyl group bonding to an aromatic ring, thereby changing the alkyl group into a different one bonding to the aromatic ring via a secondary carbon is represented by the following chemical reaction formula: 
wherein
R10 to R12 each represents an alkyl group,
X5 and X6 each represent a methyl group, an ethyl group, a halogen atom, a formyl group, a carboxyl group, an alkoxy group, a nitro group, an amino group, or a cyano group;
n is from 0 to 5.
Concretely, alkylating benzene with propylene give a main product of isopropylbenzene in which the branched alkyl group directly bonds to the aromatic ring via its tertiary carbon, but gives a minor side product of n-propylbenzene in which the non-branched alkyl group directly bonds to the aromatic ring via its secondary carbon. In the main product of isopropylbenzene, the isopropyl group is stabilized. In this, therefore, it is difficult to change the configuration of the alkyl group bonding to the aromatic ring so as to change the isopropylbenzene into n-propylbenzene. The same shall apply to any other aromatic compounds having a higher alkyl group. Anyhow, it is known that alkyl group-substituted aromatic compounds, in which the number of the branches of the alkyl group is small and/or the branched side chains of the alkyl group are short and/or the alkyl group bonds to the aromatic group via a secondary carbon, are difficult to produce. Therefore, in order to obtain n-alkyl group-substituted aromatic compounds, generally employed is a method of alkylating aromatic compounds with an n-alkyl halide or an n-alkyl alcohol. However, the method is not always satisfactory in industrial use, since the reagents to be used are expensive and since the n-alkyl group-substituted aromatic compounds produced are partly isomerized. Another method is known, which comprises alkylating toluene with ethylene in the presence of an alkali catalyst, but this is still unsatisfactory in industrial use. Given that situation, it is desired to develop efficient and inexpensive methods for producing n-alkyl group-substituted aromatic compounds.
Japanese Patent Laid-Open No. 141525/1984 discloses an inexpensive method of producing benzene compounds having an n-alkyl group from inexpensive starting materials. In the method, an alkylbenzene having a branched alkyl group, of which the number of carbon atoms is the same as that of carbon atoms of the n-alkyl group in the intended product, is contacted with zeolite catalyst along with a benzene derivative. Journal of Catalysis, Vol. 146, pp. 523-529, 1994, and Applied Catalysis A, Vol. 108, pp. 187-204, 1994 disclose vapor-phase alkylation of toluene with isopropanol and propanol in the presence of a zeolite catalyst, in which the initial-stage product of methylisopropylbenzene is trans-alkylated with the starting toluene or benzene into n-alkylbenzenes.
Halogenated aromatic compounds having an alkyl group with at least 3 carbon atoms are produced from aromatic compounds having an alkyl group with at least 3 carbon atoms through nucleophilic substitution with halogens such as chlorine and bromine. The halogenation is extremely specific to ortho (o-) and para (p-) orientation. Therefore, for obtaining meta (m-) isomer through the reaction, the product must be isomerized. The ratio of isomers of halogenated aromatic compounds having an alkyl group with at least 3 carbon atoms that are demanded in the market often differs from that of those isomers that are actually produced through halogenation. Therefore, for effectively utilizing halogenated aromatic compounds having an alkyl group with at least 3 carbon atoms, the isomerization of the compounds has an important technical meaning. Specifically, the isomerization referred to herein is to change the relative position of the halogen and the alkyl group on the aromatic ring of halogenated aromatic compounds, and does not include isomerization of the alkyl group itself. As conventional examples of isomerization of aromatic compounds, generally known are a method of using a catalyst of aluminium trichloride or the like such as that disclosed in J. Org. Chem., Vol. 27, p. 3464, 1962; and a method of using a catalyst of HF-BF3 such as that disclosed in Japanese Patent Laid-Open No. 11809/1971. Apart from those, Acta Chemica Scandinavia, Vol. B39, p. 437, 1985, and Japanese Patent Laid-Open No. 316600/1998 disclose isomerization of chloroethylbenzene with mordenite-type zeolite. Japanese Patent Laid-Open Nos. 40428/1982, 85330/1982, 163327/1982 and 309792/1995 disclose isomerization of halogenotoluenes with a catalyst of zeolite.
A compound having a higher alkyl group shall include isomers, depending on the number of the branches of the alkyl group therein, and the isomers generally have similar properties (boiling point, melting point, solubility). Therefore, it is often difficult to isolate the isomers through distillation or crystallization.
Di-substituted benzenes will be discussed. Dialkylbenzenes with alkyl groups having 1 or 2 carbon atoms have three types of isomers which are o-isomer, m-isomer and p-isomer. Of alkyl groups having 3 or more carbon atoms, however, one having 3 carbon atoms includes two types propyl groups which are n-propyl group and isopropyl group, and one having 4 carbon atoms includes four types of butyl groups which are n-butyl group, isobutyl group, sec-butyl group and tert-butyl group. Each of those alkyl group shall give three types of isomers, o-isomer, m-isomer and p-isomer, when existing in dialkylbenzenes. As a result, dialkylbenzenes with alkyl groups having 3 or more carbon atoms shall include such an extremely large number of isomers.
The boiling point difference between those aromatic compound isomers is extremely small, and a precision distillation tower having a large number of stages must be used for isolating the isomers. In that situation, it has heretofore been difficult to efficiently isolate an intended aromatic compound having an alkyl group with at least 3 carbon atoms, at high purity.
For separating chlorotoluene isomers, of which the structure differs from that of the aromatic compounds for the present invention, Japanese Patent Laid-Open No. 5155/1962 discloses an adsorptive separation method of using an X-type zeolite as the adsorbent, and Japanese Patent Laid-Open Nos. 31627/1982, 35528/1982 and 91933/1982 disclose an adsorptive separation method of using a K ion-exchanging Y-type zeolite as the adsorbent. In those methods, the adsorbents used have the ability to separate the m-isomer from the p-isomer through adsorption but do not have the ability to separate the m-isomer from the o-isomer through adsorption. In those, therefore, it is impossible to isolate m-chlorotoluene alone as the extract or raffinate component. Japanese Patent Laid-Open Nos. 131923/1983 and 176223/1984 disclose a method of separating m-chlorotoluene through adsorption, in which are used an Ag-K ion-exchanging Y-type zeolite and an Na-Cu ion-exchanging Y-type zeolite, respectively, as the adsorbent. In the method disclosed, m-chlorotoluene is isolated as the raffinate component.
In the methods noted above of processing aromatic compounds having a branched alkyl group with at least 3 carbon atoms for changing the position of the carbon atoms of the alkyl group bonding to the aromatic ring, the yield of the intended products is poor. In those, in addition, the amount of the alkyl group-substituted aromatic compounds to be recovered after the reaction is not satisfactory, or that is, the loss of the alkyl group-substituted aromatic compounds is great. Still another problem with those methods is that the activity of the catalyst used is lowered with the lapse of the reaction time. For these reasons, the methods are unfavorable to industrial use for changing the position of the carbon atoms of alkyl groups bonding to aromatic rings.
Being different from the methyl group and the ethyl group in halogenotoluenes and halogenoethylbenzenes, the alkyl group having at least 3 carbon atoms in alkyl-substituted halogenoaromatic compounds is readily dealkylated from the aromatic ring, and efficient isomerization of those halogenoaromatic compounds is difficult.
No adsorptive separation method has heretofore been known for separating specific isomers from a mixture of aromatic compounds having an alkyl group with at least 3 carbon atoms.
The object of the present invention is to provide an industrializable and highly productive method for producing aromatic compounds having an alkyl group with at least 3 carbon atoms, which comprises at least one of the following: a step of changing the position of the carbon atoms of the alkyl group bonding to the aromatic ring of an aromatic compound having an alkyl group with at least 3 carbon atoms, a step of isomerizing a halogenated aromatic compound having an alkyl group with at least 3 carbon atoms, and a step of separating an aromatic compound having an alkyl group with at least 3 carbon atoms through adsorption.
We, the present inventors have assiduously studied how to solve the problems noted above, and, as a result, have found the following: when an aromatic compound having a branched alkyl group with at least 3 carbon atoms is contacted with a zeolite-containing catalyst, then the position of the carbon atoms of the branched alkyl group bonding to the aromatic ring of the compound can be efficiently changed and, in addition, the loss of the alkyl group-substituted aromatic compound in the reaction is reduced and the catalyst activity is well protected from being lowered with the lapse of reaction time; when a halogenated aromatic compound having an alkyl group with at least 3 carbon atoms is contacted with an acid-type zeolite, then the compound can be efficiently isomerized; and when a mixture of isomers of an aromatic compound having an alkyl group with at least 3 carbon atoms is treated with an ion-exchanged zeolite adsorbent, then a specific isomer can be separated from the isomer mixture through adsorption. On the basis of these findings, we have completed the present invention.
Specifically, the invention is a method for producing aromatic compounds having an alkyl group with at least 3 carbon atoms, which comprises at least one of the following steps:
(1) a step of contacting a starting material that contains an aromatic compound having a branched alkyl group with at least 3 carbon atoms, with a zeolite-containing catalyst in a liquid phase in the presence of hydrogen therein, thereby changing the position of the carbon atoms of the alkyl group bonding to the aromatic ring of the compound;
(2) a step of contacting a starting material that contains an aromatic compound having a branched alkyl group with at least 3 carbon atoms, with a catalyst containing zeolite and containing rhenium and/or silver, in a liquid phase, thereby changing the position of the carbon atoms of the alkyl group bonding to the aromatic ring of the compound;
(3) a step of contacting a halogenated aromatic compound having an alkyl group with at least 3 carbon atoms, with an acid-type catalyst, thereby isomerizing the compound;
(4) a step of treating a mixture of isomers of an aromatic compound having an alkyl group with at least 3 carbon atoms, with a zeolite adsorbent that contains at least one exchangable cation selected from alkali metals, alkaline earth metals, lead, thallium and silver, thereby separating a specific isomer from the isomer mixture through adsorption.
According to the invention, a starting material that contains an aromatic compound having a branched alkyl group with at least 3 carbon atoms is contacted with (1) a zeolite-containing catalyst in, the presence of hydrogen, and/or (2) a catalyst containing zeolite and containing rhenium and/or silver, in a liquid phase, whereby the position of the carbon atoms of the alkyl group bonding to the aromatic ring of the compound can be efficiently changed, and, in addition, the loss of the alkyl group-substituted aromatic compound in the reaction is reduced and the catalyst activity is well protected from being lowered with the lapse of reaction time.
Also according to the invention, a halogenated aromatic compound having an alkyl group with at least 3 carbon atoms is isomerized through contact with zeolite, whereby a desired halogenated aromatic compound having an alkyl group with at least 3 carbon atoms can be efficiently obtained.
Still according to the invention, a mixture of isomers of an aromatic compound having an alkyl group with at least 3 carbon atom is treated with a zeolite adsorbent that contains at least one exchangable cation selected from alkali metals, alkaline earth metals, lead, thallium and silver, whereby a specific isomer of the aromatic compound having an alkyl group with at least 3 carbon atoms can be efficiently separated from the isomer mixture through adsorption.